Speaking of Nimbus ruins...
You can glide/fly (although glidining/climbing is much easier) to the bottom of the main tower, and be able to stand on the bottomless pit there, right in the place meant for spectators. It's been abused lately, having other people carry the flag holder into that area for safe-keeping.

During a race game, if everyone who hasn't completed the level quits, then those who have completed will have to wait for the timer to run out, despite the fact that everyone on has completed the level.

Don't know if this was posted already, but if there is an item very close to a wall, or sector (maybe even in it, but still able for the player to get it), you can go through/inside of the sector using a monitor, and you can use a ring (if respawnitemtime is 0) to go inside easily.

When you are in a cut scene in a netgame. Anyone can skip the cut scene instead of the host being able to do it only. I found this out when I was in Penopat's netgame hosting his crappy wad and while the cut scene were being played, they would suddenly go by quickly and he wasn't touching anything...

Binding something like "bind x "color etc ; skin etc ; name "Name here"" wont work right cause of the name with a space needing a "" aroudn it to make the command work be the " before the name cancels it and breaks the bind somehow.